# std::assignable_from

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< cpp‎ | concepts

Defined in header `<concepts>` |  |
template< class LHS, class RHS > concept assignable_from = std::is_lvalue_reference_v<LHS> && std::common_reference_with< const std::remove_reference_t<LHS>&, const std::remove_reference_t<RHS>&> && requires(LHS lhs, RHS&& rhs) { { lhs = std::forward<RHS>(rhs) } -> std::same_as<LHS>; }; |  | (since C++20)

The concept `assignable_from<LHS, RHS>` specifies that an expression of the type and value category specified by `RHS` can be assigned to an lvalue expression whose type is specified by `LHS`.

### Semantic requirements

Given

- `lhs`, an lvalue that refers to an object `lcopy` such that decltype((lhs)) is `LHS`,
- `rhs`, an expression such that decltype((rhs)) is `RHS`,
- `rcopy`, a distinct object that is equal to `rhs`,

`assignable_from<LHS, RHS>` is modeled only if

- std::addressof(lhs = rhs) == std::addressof(lcopy) (i.e., the assignment expression yields an lvalue referring to the left operand);
- After evaluating lhs = rhs:
  - `lhs` is equal to `rcopy`, unless `rhs` is a non-const xvalue that refers to `lcopy` (i.e., the assignment is a self-move-assignment),
  - if `rhs` is a glvalue:
    - If it is a non-const xvalue, the object to which it refers is in a valid but unspecified state;
    - Otherwise, the object it refers to is not modified;

### Equality preservation

Expressions declared in requires expressions of the standard library concepts are required to be equality-preserving (except where stated otherwise).

### Notes

Assignment need not be a total function. In particular, if assigning to some object `x` can cause some other object `y` to be modified, then x = y is likely not in the domain of `=`. This typically happens if the right operand is owned directly or indirectly by the left operand (e.g., with smart pointers to nodes in a node-based data structure, or with something like std::vector<std::any>).

### Example

Run this code

    #include <atomic>
    #include <concepts>
    #include <string>

    int main()
    {
        // Normal basic usage, checks lvalue reference assignment
        static_assert(std::is_assignable_v<int&, int>);
        static_assert(std::assignable_from<int&, int>);

        static_assert(std::is_assignable_v<std::string&, std::string>);
        static_assert(std::assignable_from<std::string&, std::string>);

        // Fundamental types don't support assignment to an rvalue
        static_assert(!std::is_assignable_v<int, int>);
        static_assert(!std::assignable_from<int, int>);

        // std::assignable_from doesn't accept all valid assignment expressions:

        // rvalue reference assignment
        static_assert(std::is_assignable_v<std::string&&, std::string>);
        static_assert(!std::assignable_from<std::string&&, std::string>);

        // rvalue assignment
        static_assert(std::is_assignable_v<std::string, std::string>);
        static_assert(!std::assignable_from<std::string, std::string>);

        // std::atomic::operator= returns by value
        static_assert(std::is_assignable_v<std::atomic<int>&, int>);
        static_assert(!std::assignable_from<std::atomic<int>&, int>);
    }

### References

- C++23 standard (ISO/IEC 14882:2024):

  - 18.4.8 Concept `assignable_from` [concept.assignable]

- C++20 standard (ISO/IEC 14882:2020):

  - 18.4.8 Concept `assignable_from` [concept.assignable]

### See also

is_assignableis_trivially_assignableis_nothrow_assignable (C++11)(C++11)(C++11) | checks if a type has an assignment operator for a specific argument (class template)

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